The electrostatic coating process for the application of color coatings on various substrates is well known. This process uses a composition that is an electrically chargeable powder mixture of a film-forming polymer and a pigment (or pigments) having the desired color. Unfortunately, preparation of such powder coating compositions for application by a powder spray applicator has presented a problem in the past in connection with platy pigments.
Pearlescent or nacreous pigments simulate the effect of natural pearl and are composed of thin platelets that are transparent in the visible region of the spectrum. The platelets are very smooth and part of the light that strikes the platelets is reflected and part is transmitted through the platelets. That part of the light that is transmitted is subsequently reflected by other layers of platelets. The result is that multiple reflections from many layers occur and this results in depth of sheen since the eye cannot focus on one particular layer.
The reflection that occurs is specular in that the angle of incidence equals the angle of reflection. The amount of light reflected at non-specular angles is small and the amount of light reflected diminishes very quickly as the specular angle is passed. The result is that pearlescent pigments are extremely sensitive to viewing angle. In order for the maximum amount of light to be reflected, the platelets must be extremely smooth. Any surface roughness causes light to be scattered in a non-specular manner and diminishes the lustrous effect.
The platelets must be aligned parallel to each other and to the substrate for maximum reflectivity. If not so aligned, light will be reflected randomly and again, luster will diminish. The amount of light that is reflected depends on the index of refraction. As the index of refraction increases, the amount of reflected light increases.
Platy pigments of a metallic material rely on their laminar structure for maximum appearance effect. Such plates include, for instance, metallic flakes such as aluminum, bronze and stainless steel plates as well as natural or synthetic pearlescent pigments exemplified by, e.g., natural pearlescence or a metal oxide-coated substrate such as titanium dioxide-coated mica, iron oxide-coated mica, titanium dioxide-coated glass, iron oxide-coated glass, and iron-coated aluminum flakes. The laminar structure of such metallic or pearlescent pigments is destroyed during the extrusion or grinding processes, which are used in the manufacture of powder coating compositions and as a result, the coating appearance achieved exhibits a reduced luster effect.
The industry has attempted to avoid the foregoing problem by dry blending the pigments and polymer powder, i.e., the polymer carrier is mechanically blended with the pigment. Unfortunately, the pigment and the powder particles usually develop a different charge magnitude, which results in application issues and color shift when the powder coating composition is electrostatically sprayed on a substrate. In addition, the pigments have a tendency to separate from the polymer powder in that volume of the powder spray composition, that does not attach to the substrate and is then recovered. As a result, the recovered oversprayed material is difficult to reuse.
To overcome the problems encountered as a result of the dry blending process, a blending process in which the powder base and the pigment were mixed and then heated to a temperature sufficient to soften the surface of the powder particles so that the pigment could bind to the surface of such particles was developed, as described in U.S. Pat. No. 5,187,220. While this process works well for corona electrostatic charging and alleviates the problems with reusing the powder over spray, it is a rather costly process.
A significant advance in the art is described in U.S. Pat. No. 5,824,144, the disclosure of which is hereby incorporated by reference. The described metal containing platelet pigment is provided with a viscous surface layer of polymer or other sticky liquid material. When that treated pigment is blended with powder coating composition, the powder attaches to the surface of the pigment thereby minimizing color separation. The pigment is also encapsulated by the powder to thereby present a single surface. A better charge of those pigment particles which do not become attached to the powder materials is also realized.
A significant problem in connection with the use of metal-containing platelet pigments in a powder coating composition is that the concentration of the pigment which can be incorporated is limited. While pigment amounts of up to about 10% or more can be utilized in single application environments or in the laboratory, the concentration is limited to about 3% when operating on a larger scale as is required commercially. At levels higher than about 3%, numerous problems arise. These include spray gun clogging and tip build-up, blotchy panel appearance and color separation.
It has been discovered that a treatment which had previously been developed for pearlescent pigments intended for exterior use to provide improved humidity, resistance and overall weatherability and which was used in liquid coating systems such as a solvent or water borne automotive paint systems, for example, pigmented base coat and clear top coat, provides superior properties in the context of powder coatings. The exterior pigment treatment surprisingly improves the application properties of the pigment when incorporated into powder coatings. Such improvements include less build up of the pigment at the electrostatic gun tip, improved transfer through feed lines, improved transfer efficiency of the pigment and a more uniform coating appearance. Commonly assigned U.S. Pat. No. 6,524,661 teaches an improved pearlescent pigment powder coating composition comprising a mixture of a particulate resin carrier and a particulate pigment in which the pigment is a laminar metal-containing pigment having a first coating of a hydrated aluminum oxide or hydrated cerium and aluminum oxides and a coating of a hydrolyzed silane coupling agent or intermingled with the first coating. A Scanning Electron-imaging Microscopy (SEM) image of such a product is shown in FIG. 4. The SEM image shows titanium oxide coated mica platelet with the silane surface treatment at a magnification of 5,500× and 33,000×. The powder coating mix consisting of the exterior treated pigment has the advantage of minimizing the pigment separation at the gun tip, but affects the quality of the film of the coated part once powder is cured. We believe that the aluminum hydroxide is not present on the outer surface of the silane coating. See also U.S. Pat. No. 6,176,918. Thus, due to the surface energy differences between the silane coated pearlescent pigment and resin, the cured powder coated film has a textured or orange-peel-like appearance.
It is known in the industry to add aluminum oxide to powder coating compositions by either preblending the aluminum oxide with resin and then mixing the preblend with pearlescent pigment or simultaneously mixing aluminum oxide, resin, and pearlescent pigment together. See Nargiello, Fumed Metallic Oxides Improve Powder Processing and Application, Volume 4(3), pages 16-20 (June 1993). As shown in Comparative 5 below, spraying the simultaneously made mixture onto a panel did not provide any appearance improvement.